Pressure sensitive adhesives (“PSAs”), as commonly understood in the art and used herein, are typically viscoelastic materials that adhere instantaneously to most substrates with the application of slight pressure and remain permanently tacky. PSAs are finding increasingly more challenging uses as they become stronger and easier to apply. Such PSAs have now found many uses in industrial applications and are increasingly being favored by automobile manufacturers for mounting various parts of an automobile, from dashboard parts to body panels.
Low surface energy (“LSE”) materials such as polypropylene and steel coated with LSE paints are increasingly used in the manufacture of automobiles. LSE materials also are increasingly used in consumer electronic applications. In the automotive industry, for example, parts such as body side moldings and weather seal foams are bonded to the substrates, typically polypropylene and painted steel, through the use of such PSAs. Clearly, such applications require not just high cohesive strength but high adhesion to LSE/non-polar surfaces as well as resistance to elevated temperatures and good ultraviolet (“UV”) light resistance and aging properties.
Unfortunately, substrates composed of LSE materials such as polypropylene, polyethylene, and new-generation automotive paints and clear coats are extremely difficult to adhere to, particularly with PSAs. The main factors engendering this difficulty are the surface energies of the PSAs (such as acrylics), which are higher than those of the substrates (and thereby, the contact area between the PSAs and substrate surface is limited because the PSAs cannot readily wet the surface of the substrate), and a general lack of specific loci for covalent or strong non-covalent bonding (e.g., hydrogen or ionic bonding) on the surfaces, so that adhesion must occur primarily through the weaker van der Waals forces.
Common strategies used in the art to obtain satisfactory adhesion to LSE substrates include lowering the surface energies of the PSAs through appropriate choice of polymer composition or additives such tackifiers, decreasing the modulus of the composition through decreased crosslinking and/or heavy use of tackifiers, and changing the nature of the surface of the LSE substrates through use of relatively more polar primers. The first two strategies generally lead to unsatisfactory results because the cohesive strength of the PSAs typically suffers, i.e., decreases, due to both tackification and decreased crosslinking, and there is a progressively increasing market need for PSAs that simultaneously possess high cohesive strength and high adhesion. The last strategy can yield satisfactory results with respect to PSA performance, but requires additional primer coating and drying steps to be added to the process.
The use of macromers with PSAs and for other applications are known. Macromers are relatively low molecular weight polymers having a functional reactive group at one or more terminals of the polymer. Patents that disclose such PSAs include, and are all incorporated by reference herein in their entireties, the following U.S. Pat. Nos. 3,786,116, 4,551,388, 4,554,324, 4,693,935, 4,732,808, 4,833,179, 4,851,278, 5,006,582, and 5,057,366.
Another approach known in the art involves the tackification of acrylics to improve adhesion to low-energy surfaces such as polypropylene or polyethylene. Patents that disclose such PSAs include, and are all incorporated herein by reference in their entireties, the following U.S. Pat. Nos. 4,418,120, 4,726,982, 4,988,742, and 5,028,484, and European Publication No. 303430. All of these patents, incorporated herein by reference in their entireties, deal with tackification of the acrylic phase in order to improve adhesion properties. A number of these patents necessitate the use of a UV, on-web polymerization process. Because some use rosin ester type tackifiers, they would be expected to have poor UV and oxidative stability.
Yet another approach that is known in the art utilizes acrylic polymers and elastomers in two-phase systems, some of which claim improved adhesion to painted surfaces and low temperature performance. Representative patents include the following U.S. Pat. Nos. 4,243,500 and, 5,024,880, and European Publication Nos. 349216 and 352901, all of which are incorporated by reference herein in their entireties. These patents involve using a UV-polymerization process to obtain a two-phase network, as UV light is essential to achieve crosslinking between the acrylic and the unsaturation in the elastomer. Another representative patent is U.S. Pat. No. 5,143,972, which is incorporated by reference herein in its entirety.
Grafting a saturated hydrocarbon macromer onto an acrylic backbone will yield a two-phase compound consisting of a graft (or comb-type) copolymer evincing all of the desirable qualities mentioned above. These compounds are described in commonly owned U.S. Pat. No. 5,625,005, which is incorporated by reference herein in its entirety.
U.S. Pat. Nos. 6,670,417 and 6,642,298, which are incorporated by reference herein in their entireties, claim to improve upon the compounds disclosed in U.S. Pat. No. 5,625,005 (“the '005 patent”) by providing further specific working examples within the framework set forth in the '005 patent.
Within the context of automobile manufacturing and similar applications, a particularly popular use of PSAs is for bonding of body side molding parts and weather seal rubbers to painted surfaces. The materials to be bonded, i.e., thermoplastic polyolefins (“TPOs”), rubbers and automotive paints, are LSE materials. PSAs used to bond these materials are therefore invariably tackified and/or plasticized. Tackifiers and plasticizers (modifiers) are, for the most part, low molecular weight materials that are not covalently bonded to the PSA polymer. The modifiers have a two fold beneficial impact on the PSA material characteristics because they increase the bond strength that can be obtained with the PSAs.
On one hand, the modifiers lower the surface energy of the PSAs so that the surface energies of the PSAs are better matched to those of the substrates. Such matching of the surface energies ensures that the PSAs will tend to spontaneously wet out the substrate, thereby increasing the contact area. The PSA performance modifiers invariably consist of materials with solubility parameters similar to those of the substrates. Because the substrates to be bonded consist of LSE materials such as TPOs and painted surfaces, the PSA modifiers consist for the most part of low-solubility parameter materials such as pure hydrocarbons and silicones. The modifiers also alter the rheological or viscoelastic characteristics of the PSAs. They decrease the modulus of the PSAs so the materials tend to flow better, thereby increasing contact area in a given amount of time, and they modulate the frequency positions where the PSAs can best dissipate the energy applied during debonding events.
In the context of foam constructions, use of PSA performance modifiers can have negative consequences, in the instances where the solubility parameters of the foam materials are similar to those of the PSA performance modifiers. The PSA additives tend to migrate into the foams because the tackifier and/or plasticizer molecules are not covalently bonded to the PSA, the modifiers possess relatively low molecular weights, and the modifiers possess solubility parameters similar to those of the foams. This migration has the following two negative consequences: it depletes the PSA of the performance-enhancing additives and alters the foam performance characteristics (i.e., it tackifies and/or plasticizes the foam). In addition, processing aids and foaming agents (blowing agents) are often used in the manufacture of foams. These are typically non-covalently bonded low molecular weight materials. In adhesive-foam-adhesive constructions, the processing aids and foaming agents can migrate from the foams into the adhesives where they can potentially induce deleterious changes of the adhesive characteristics by chemically modifying the materials or by blooming to the interface of the adhesive and the substrates and thereby forming weak boundary layers.
What is needed is further enhancement of the desirable properties evidenced by PSAs, including high cohesive strength and the ability to strongly bond to LSE surfaces such as polypropylene and automotive paints without the use of primers, and preferably with reduced costs. It is also highly desirable to eliminate or mitigate the migration of materials from adhesives into foams and vice versa. By doing so, stable, strong bonds can be obtained and maintained between the substrates and the adhesives. The embodiments of the present disclosure answer these and other needs.